The present invention relates to an apparatus for handling a continuous length of coiled tubing for insertion into or removal from a well bore, such apparatus is commonly referred to in the art as a coiled tubing injector.
Coiled tubing injectors of the type described in U.S. Pat. No. 4,585,061 were originally used during workover operations to inject a relatively small diameter, continuous length of coiled tubing into a well bore while the well was under pressure. In such a case, the tubing must be literally forced or "injected" into the well through a sliding seal to overcome the well pressure until the weight of the tubing exceeds the force produced by the pressure acting against the cross-sectional area of the tubing. Thereafter, the weight of the tubing has to be supported by the injector. The process is reversed as the tubing is removed from the well. In recent years, the coiled tubing has been used in combination with a mud turbine motor to drill original bores, has been used as the permanent tubing in production wells, and continues to be used in various workover and service applications. Because of the advantages of continuous coiled tubing, and the resulting new uses, the state of the art of manufacturing coiled tubing has rapidly progressed until tubing is almost three inches in diameter. These large tubings have a wall thickness and sufficient tinsel strength to support up to 20,000 feet hanging in a well bore.
The only method by which a continuous length of tubing can be either forced against pressure into the well, or supported while hanging in the well bore, while lowered or raised is by continuously gripping the tubing along its length. This is achieved by arranging continuous chain loops on opposite sides of the tubing with active reaches extending parallel to the tubing just before it enters the well bore. The continuous chains carry a series of gripper shoes which are pressed against opposite sides of the tubing and grip the tubing.
In order to handle progressively larger, longer, and heavier tubing, the gripping force must be progressively increased. This can be achieved by increasing the force pressing the gripper shoes against the tubing, by increasing the number of gripper shoes by increasing the length of the chains, by increasing the contact area of the gripper shoe, or by improving the gripping surfaces.
As the length of the chain increases, the tolerance problems to insure that all of the individual grippers are contacting the tubing with equal force presents a practical limitation, which has been reached with current designs. The application of greater force to press the grippers against the tubing, is an even more clear limitation because this will deform. Current grippers are already surfaced with carbide grit which penetrates the surface of the tubing to the point of damaging the tubing. The use of carbide surface treatment is relatively extensive and the expensive chain must be replaced frequently because when the grit wears smooth, the grippers can no longer effectively handle the tubing. The grippers can encircle only a limited percentage of the circumference of the tubing because the grippers must engage and disengage from the tubing at the beginning and end of the active reaches of the chains, and any attempts to increase the circumferential contact has resulted in unacceptable marring of the surface of the tubing to the point of causing the tubing to fail. Any increase in the force applied to the gripper results in unsatisfactory deformation of the tubing, typically causing it to become permanently egg-shaped. All of these design variables have reached the practical limits using current designs for moderately sized tubing, and the larger tubing cannot be satisfactorily handled, except in relatively shorter lengths, with even larger and longer tubing presently being demonstrated.